1. Field of the Invention
The present invention relates to a signal processing apparatus and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus such as a color photocopier based on electrophotographic technology, an image is formed by superimposition of toner images of Y (yellow), M (magenta), C (cyan) and K (black) colors. The toner images of these colors are developed on photoreceptor drums of those colors. The toner images of these colors are sequentially transferred onto the annular belt as an intermediate transfer member, and the transferred images are transferred onto recording paper.
In one of the conventional structures, a predetermined density patch and registration mark (pattern) are formed on the intermediate transfer member, and the image is detected by an optical sensor. Based on this detection signal, density and color registration are corrected. In the step of color registration correction, the position of the registration mark of each color is detected by a sensor. Based on the amount of misregistration, the main scanning correction volume, sub-scanning correction volume, overall lateral magnification correction volume, partial lateral magnification correction volume, and skew correction volume (scanning line inclination correction volume) are calculated, whereby color misregistration is corrected.
According to a conventional method (e.g., Patent Document 1), in the color registration correction, the detection signal from the sensor is binarized. The position of the binary data is estimated from the timed interval of writing the registration mark on the intermediate transfer member, and the registration mark is detected within the estimated range plus α. The outside of this range is sequentially masked, thereby removing noise caused by a scratch or dust on the intermediate transfer member.
A color registration correction technique has been proposed. According to this proposal, the detection signal from a sensor is filtered by an IIR (infinite Impulse Response) type, FIR (infinite Impulse Response) type and moving average type digital low-pass filters, thereby removing the noise resulting from a scratch or dust on the intermediate transfer member (e.g., Patent Document 2).
[Patent Document 1] Japanese Non-Examined Patent Publication 2001-265086
[Patent Document 2] Japanese Non-Examined Patent Publication 2003-98791
However, the method of removing noise by detection of the registration mark within the conventional predetermined range depends on the signal within the predetermined range to detect the registration mark including the noise component having occurred thereto. This factor has been beyond control because of chronological changes even if there is no problem in the initial period.
If a simple structure is used in the conventional method of removing noise by a low-pass filter, there is concern about the possibility of ensuring accurate detection of the registration mark.
FIG. 14 (a) shows the gain of the frequency in the low-order FIR filter. FIG. 14 (b) shows the phase component of the frequency in the low-order FIR filter. FIG. 15 (a) shows the gain of the frequency in the high-order FIR filter. FIG. 15 (b) shows the gain of the phase component in the high-order FIR filter. As shown in FIGS. 14 (a) and (b), when the FIR filter of a low order is implemented, the filter characteristics are adversely affected. As shown in FIGS. 15 (a) and (b), if the high-order FIR filter is implemented, a delay in response to the order occurs to the waveform although the filter characteristics are stabilized. This has an adverse effect on the precision in position detection. Such problems have been left unsolved in the conventional art.
The IIR filter improves the filter characteristics by feedback. However, this may lead to a system of poor stability, depending on the design. This makes it necessary to keep track of the chronological changes of an object before designing. Such a problem has been left unsolved in the conventional art.